4S6TMP.com ā Low-Pass, High-Pass, Band-Pass, Band-Stop | Butterworth, Chebyshev, Bessel
Passive LC filters use inductors and capacitors to shape the frequency response of a signal. Unlike active filters (which use op-amps and require power), passive LC filters can handle high power, work at RF frequencies, and introduce no noise or distortion.
Butterworth response provides the flattest possible passband with no ripple. It's the best choice when you want maximum flatness and don't care about phase linearity.
Chebyshev response allows ripple in the passband in exchange for a steeper rolloff. A 0.5dB ripple Chebyshev has a much sharper transition than Butterworth of the same order. Good when you need strong attenuation close to the passband.
Bessel response maximizes phase linearity (constant group delay), which means it preserves the shape of pulses and modulated signals. The tradeoff is a gentler rolloff. Best for data communications and digital signals.
Normalized element values (g-values) for low-pass prototypes with 1 rad/s cutoff and 1Ī© termination:
To convert to actual component values: L = g Ć Zā / (2Ļ Ć fā) and C = g / (2Ļ Ć fā Ć Zā)
What's the difference between Pi and T topology? Both implement the same filter response but with different component arrangements. Pi networks have shunt elements at the input and output, which can be easier for impedance matching. T networks have series elements at the input and output.
Can I use standard component values? The E12 series (12 values per decade) is usually sufficient for most filter designs. For critical applications, use E24 or E96 series. This tool shows the nearest standard values.
What about parasitic elements? Real inductors have parasitic capacitance and resistance, and real capacitors have parasitic inductance and resistance. These limit the practical performance above ~100 MHz. For VHF/UHF, consider using transmission line filters instead.